Try not to beat yourself up over what may have happened..I too did for 4 days over the why and how and then a friend of mine said this to me (maybe it will help you too)
"she had an incident, heart, seizure or such..she will not be able to cross over until YOU accept she had a heart attack..she can't leave this earth watching you in such sorrow..so put it to rest that she did have a heart attack,and move past that part of it,for her,for you,for your kids.. Grieve, cry,etc,but leave the "how" behind.Easy for me to say..I know how awful this is..It is not only emotional but also very physical..Stay strong my friend..start talking about all the funny things she did!!"
Some how that writing from my friend did it, I mean don't get me wrong Im still shedding tears and looking for her in all the places she would lay but I feel at peace and I started to hurt more knowing that maybe I'm keeping her spirit here next to me with all my sorrow and once I let go, I was Finally able to look at pictures of her and it took me 4 days before I would go out into my back yard where she was buried..My heart is still heavy but I m at peace. I hope you will find your Peace
Oh one more thing I finally called the vet "again hoping for the HOW answer" and he said most of the sudden deaths are caused by heart attacks or seizure ....